Another theme could be growth. Through this kiss, miyu and kanata might experience personal growth as they become more open and vulnerable in their relationship. They might start to see each other in a new light and grow as individuals as well as a couple.
The theme of acceptance could also be present. Maybe they have been hiding their feelings due to various reasons, and the kiss is a sign of them accepting their love for one another and being ready to face any consequences or challenges that come with it.
I haven't actually read this specific fanfiction about miyu and kanata kissing. But generally, such a fanfiction might explore the relationship between the two characters in a romantic way. It could show their deepening feelings through this kiss, perhaps as a turning point in their relationship.
Often, miyu is written as someone who is very in tune with her emotions. This can lead to some really interesting character interactions in fanfics, especially when paired with kanata. Kanata may be seen as a bit more of a mystery, and fanfic authors might explore his backstory and what makes him tick in relation to miyu. For example, in some fanfics, his past experiences could be what shapes his relationship with miyu, whether it's through shared traumas or simply different outlooks on life that they gradually come to understand about each other.
In the fanfiction, their relationship could develop further by sharing their dreams and fears. Maybe one night, under the stars, they start opening up about what they are really afraid of and what they hope for in the future. This emotional vulnerability would create a stronger bond between them. Also, if they start to teach each other new skills, like Kanata teaching Miyu how to better control her UFO baby powers and vice versa, it would also enhance their relationship.
One interesting plot idea could be that Kanata and Miyu discover a new power related to their UFO baby forms. For example, they might be able to communicate with an ancient alien race that left behind clues on Earth. Another idea is that they have to deal with a new villain who is also after the secrets of their UFO baby origins. This villain could be causing chaos in their town, and Kanata and Miyu have to use their unique abilities to stop him.
Rather than focusing on lemon fanfics, you might explore regular miyu and kanata fanfics that focus on their relationship development, adventures they go on together, or their character growth. These can be found by using the search functions on relevant fanfic platforms and filtering for the appropriate tags.
One common theme is loyalty. Miyu often has to decide where her loyalties lie, whether it's with the Akatsuki or with other characters she meets in the Naruto world.
One possible theme could be love and acceptance. Their kiss could symbolize the acceptance of each other's differences. Another theme might be growing up. As they share this intimate moment, it could represent a step in their journey of growing up and understanding more complex emotions.
One possible theme could be the discovery of new feelings. For example, perhaps Charlie and Jason are friends at first and the kiss is a sudden realization of deeper romantic emotions.
A theme might be about forbidden love. Their relationship as siblings makes a kiss between them something that goes against the norm, so the fanfiction could delve into the struggle and emotions that come with that. It could also explore themes of acceptance or rejection within the family and among friends when such a relationship is revealed.
A common theme might be the exploration of forbidden love. If in their fictional world there are some sort of rules or circumstances that make their relationship difficult, the kiss could be a symbol of defying those odds. Also, themes of acceptance could be present. They may have to deal with how others in their world view their relationship after the kiss, and the story could focus on them finding acceptance within their community or among their friends.